Polyimide resins obtained by heating a resin composition containing a polyimide precursor to imidize it have been widely used as insulating films (resin films) in semiconductors and circuit boards.
In recent years, attempts have been made to pattern a resin film consisting of a photosensitive polyimide resin by exposure, but it is difficult to pattern widely used aromatic polyimides by exposure because they are significantly colored by intramolecular conjugation and charge transfer complexation so that they have very low light transmittances over a wide wavelength range from UV to visible regions.
Polyimide precursors before imidization have higher light transmittances than after imidization. However, resin films having a film thickness of 10 μm or more as used in circuit boards cannot be precisely patterned with g rays (wavelength 436 nm) or i rays (wavelength 365 nm) commonly used for patterning because of the insufficient light transmittances of precursors of aromatic polyimides.
Moreover, polyimide precursors are more soluble in developing solutions than polyimide resins so that not only exposed parts but also unexposed parts may be dissolved during the developing step. In any event, it was difficult to precisely pattern resin films consisting of polyimide resins by exposure and development.
The present invention was made to overcome the disadvantages of the prior art described above with the purpose of providing a resin composition from which a precisely patterned film can be formed even when it has a large thickness.